1. Field of the Invention
The present invention relates to an alignment method and apparatus of mask patterns when the mask patterns are transferred onto a wafer using an exposure apparatus.
2. Description of the Related Art
A photolithography process performed when manufacturing a thin-film device such as a thin-film magnetic head, a micro device, a semiconductor device, a liquid crystal display device and the like includes a step of transferring a mask pattern onto a wafer using an exposure apparatus such as, for example, a step of projecting and transferring a mask-pattern image onto each shot area by using a stepper. In such mask-pattern transferring step, it is required to accurately carry out alignment between a pattern on the mask called a reticle and a shot area on the wafer.
Hereinafter, described will be a conventional alignment method in case that patterns of a plurality of layers are stacked with each other, that is, for example, in case that a pattern of a first layer is formed on a wafer and a pattern of a second layer is stacked and formed on the first layer.
In such conventional method, alignment for the pattern of the second layer was performed with reference to an alignment mark of the first layer. However, a pattern arrangement state of the first layer might be in most cases deviated from a reference pattern arrangement state because of various processes might be performed after the exposure of the first layer. Therefore, in the conventional method, the coordinates of the alignment mark of the first layer were measured and the alignment for the pattern of the second layer was compensated depending upon the amount of deviation from absolute coordinates of the measured coordinates.
However, according to the conventional alignment method, because the amount of deviation of the first layer was measured and then the alignment was compensated using the measured deviation of the first layer when forming the second layer pattern, it was impossible to form the second layer pattern with high pattern arrangement precision. This is due to the fact that a state of an optical system in the exposure apparatus was deficient in stability when the second layer pattern was formed. Particularly, when the optical system of the exposure apparatus was adjusted and the amount of deviation was compensated, it took a long time until the system was stabilized, so that it was quite difficult to perform exposure in a fully stable state.